There are many different ways of sitting on the floor of course - legs crossed, legs tucked under; legs to the side; legs outstretched; one leg bent, one leg straight; squat; etc. And it is a good idea to change position every few minutes.
Floor sitting may not be easy for everyone so it is worth using props such as a folded blanket, a yoga block, a cushion or a low stool. The main thing is to make sure that the pelvis is "untucked". You should see or feel a bit of a curve in the lower back. If the lower back is rounded, then the pelvis is tucked under. The best way to get the feel of an untucked pelvis is to go on all fours and let the back (and abdomen!) relax down. It feels sooo good!
Be sure to respect the current boundaries of your body's capability. Only lower yourself as much as you are comfortably able at the present time. There is no prize for how low you can go! If you make floor sitting a daily habit, you will naturally be able to lower yourself further and further. Take your time, there is no rush! Most of all, have fun with it! Have a meal sitting around the coffee table, have an indoor picnic, bring the laptop to floor level, play a board game, read a book, peel vegetables, etc. The possibilities are endless and so are the benefits. Try it and see.
It's also worth remembering that there is not ONE perfect sitting position. Ideally, you will change position every 10 to 15 minutes - sitting in many different ways, standing, lying down, moving your neck, moving your legs, moving your arms, going for a walk, etc. Variety of movement is essential for the health of your joints and soft tissues!